An adaptive filter is used to remove the maternal ECG signals from abdomenally measured signals. The fetal heart rate (FHR) can be determined by examination of the residue. Means dependent partly on correlation are used to extract the FHR even when the noise level is very high. The noise itself is greatly reduced by use of electrodes that are very large and by using very low noise amplifiers. The procedure is highly mathematical but the algorithms have been designed so that they can be done in real time using microprocessors system.